1. Field of the Invention
The invention relates to a new process for the continuous preparation of dimethyl carbonate (DMC), characterized in that carbon monoxide and methyl nitrite are reacted with one another in the gas phase in the presence of a heterogeneous catalyst and the dimethyl carbonate thereby formed is isolated in subsequent process steps. The process according to the invention is particularly suitable for the industrial preparation of dimethyl carbonate.
Dimethyl carbonate is an important starting material for the preparation of aromatic polycarbonates. It is used furthermore as a starting material for the synthesis of aliphatic and aromatic mono- and diisocyanates, as a methylating agent, as a substitute for toxic phosgene in the preparation of pharmaceutical and agrochemical products, as a solvent and an agent for improving the octane rating of carburettor fuels, and as an intermediate product in the preparation of synthetic lubricants.
2. Description of the Related Art
The reaction between carbon monoxide and methyl nitrite, which is based on the formation of dimethyl carbonate, can be described by reaction equation (1). ##STR1##
Methyl nitrite itself can be produced for this in a manner known per se in a prior reaction in accordance with one of reaction equations (2) to (5). EQU 4 NO+O.sub.2 +4 CH.sub.3 OH.fwdarw.4 CH.sub.3 ONO+2 H.sub.2 O (2) EQU NO+NO.sub.2 +2 CH.sub.3 OH.fwdarw.2 CH.sub.3 ONO+H.sub.2 O (3) EQU N.sub.2 O.sub.4 +CH.sub.3 OH.fwdarw.CH.sub.3 ONO+HNO.sub.3 ( 4) EQU 2 NaNO.sub.2 +H.sub.2 SO.sub.4 +2 CH.sub.3 OH.fwdarw.2 CH.sub.3 ONO+Na.sub.2 SO.sub.4 +2 H.sub.2 O (5)
The preparation of dimethyl carbonate by reaction of carbon monoxide and methyl nitrite in the gas phase in the presence of a heterogeneous catalyst, which is preferably a platinum metal catalyst fixed to a support, has been described in various instances, for example in the following scientific publications, Offenlegungsschriften and patent specifications:
JP 60 181 051, X.-Z. Jiang et al.; Cuihua Xeubao 10 (1) 75-78 (March 1989), EP 425 197, X.-Z. Jiang; Platinum Metals Rev. 34 (4), 178-180 (1990), EP 464 460, EP 503 091, EP 501 507, EP 503 618, EP 523 508, EP 523 728 and EP 538 676, EP 559 001, EP 558 996, EP 559 212, EP 565 076 and EP 581 240.
Apart from the European Patent Application EP 523 728 cited, none of the publications listed describes a route which would be suitable for continuous industrial preparation of dimethyl carbonate. Thus, the reaction products obtained are in general mixtures in which, in addition to dimethyl carbonate, the desired target compound, other substances are present, such as, for example, dimethyl oxalate, which is formed in the course of a side reaction which proceeds according to reaction equation (6), methyl formate, formaldehyde dimethyl acetal, water and, in particular, methanol. However, such mixtures are completely unsuitable for many of the possible intended uses of dimethyl carbonate. ##STR2##
A continuous process procedure, which is to be aimed for from industrial aspects, must basically correspond to the principle illustrated in FIG. 1, and hence a circulatory process. It must be designed such that the nitrogen oxides obtained in the course of the formation of dimethyl carbonate in accordance with reaction equation (1) and all the other gaseous and condensed products, by-products and auxiliaries either are recycled completely or for the most part into the process, if they can be used or do not adversely impair the economics or the industrial safety of the overall process, or, if these components adversely impair the economics or the industrial safety of the overall process, or are removed from the circulation completely or to the extent necessary for unimpaired continuous operation of the overall process.
EP 523 728 describes a process, the principle of which is illustrated in FIG. 2 and which comprises the continuous preparation of dimethyl carbonate by reaction of methyl nitrite with carbon monoxide in the gas phase over a heterogeneous catalyst, which is preferably a platinum metal contact catalyst fixed to a support, and subsequent isolation of the dimethyl carbonate, obtained as a mixture with methanol, dimethyl oxalate and other impurities, in the course of an extractive distillation in which dimethyl oxalate is used as the extraction agent. EP 523 728 furthermore comprises recycling of the nitrogen oxides liberated in the course of the reaction of methyl nitrite with carbon monoxide, together with the unreacted gaseous reaction partners and the additional gas necessary for rendering the system inert, preferably nitrogen, into a process step which precedes the actual preparation process of dimethyl carbonate and corresponds to reaction equation (2), and in which the methyl nitrite required for the reaction is formed again by feeding in methanol and oxygen and removing to the greatest extent the water thereby liberated. This is thus a circulatory process with respect to the gaseous components participating, that is to say with respect to the inert gases and auxiliaries, the unreacted gaseous reactants, such as, for example, the unreacted methyl nitrite and carbon monoxide, and the nitrogen oxides participating.
The following points are disadvantages of the process described in EP 523 728 which jeopardize its industrial applicability from the economic and ecological aspect:
Large amounts of auxiliaries must be circulated for the preparation, isolation and purification of the dimethyl carbonate. For example, taking the information from Example 1 of the patent application cited as a basis, the following amounts to be circulated per kg of DMC result:
4.8 kg of methanol, PA1 6.0 kg of dimethyl oxalate, PA1 2.8 kg of methyl nitrite, PA1 1.6 kg of carbon monoxide, PA1 1.2 kg of nitrogen monoxide and PA1 7.8 kg of nitrogen PA1 (a) carbon monoxide and methyl nitrite are reacted in the gas phase in the presence of a heterogeneous catalyst comprising a platinum metal, preferably a supported catalyst comprising palladium, and an inert gas in the temperature range from 50.degree. to 170.degree. C., preferably from 70.degree. to 150.degree. C., and in the pressure range from 1 to 5 bar, preferably 2-4 bar, whereby, as an activator, hydrogen halide, halogen, methyl chloroform and/or other substances which contain halogen acting activating under the reaction conditions in a concentration of 0 to 3,000 ppm, preferably 10 to 1,000 ppm, is added to the gas mixture. PA1 (b) the mixture obtained in (a) is separated into gaseous and liquid reaction products, a part of the gaseous stream of from 0 to 7% by weight, preferably 0.1 to 5% by weight is removed, the therein contained low-boiling constituents are separated off and directed to a further work-up, the therein contained nitrogen monoxide is converted with oxygen and methanol to yield methyl nitrite, which methyl nitrite is separated off and recycled to the process, and the remaining accumulated inert gases are excluded from the process, PA1 (c) the gaseous products are reacted with methanol, oxygen and if appropriate freshly added nitric oxide or nitric oxide equivalents for renewed formation of the methyl nitrite, the gas mixture which contains the newly formed methyl nitrite being led off and recycled to the preparation of dimethyl carbonate, and water and any other liquid by-products formed also being led off and removed from the circulation, preferably after subsequent recovery of the useful substances contained therein, and PA1 (d) the liquid products from (b) are subjected to separation by distillation, in which the entire product mixture is initially subjected to a first distillation, which is carried out under a pressure of 1 to 25 bar, preferably 1 to 12 bar, and then either PA1 (f) pure dimethyl carbonate is obtained by distillation of the mixture obtained as the bottom runnings of the first distillation carried out under increased pressure.
In particular, because of the large excess of methanol which is fed into the methyl nitrite reactor 3 of FIG. 2 and is about 500% of the amount required stoichiometrically, a quite considerable distillation expenditure is necessary, which leads to high energy costs if the unreacted content of methanol is to be recovered from the bottom discharge of the reactor.
Since recycling of these auxiliaries and secondary components (water, nitric acid, methyl formate, formaldehyde dimethyl acetal) requires removal by distillation from the dimethyl carbonate, the desired reaction product, above all in respect of methanol and dimethyl oxalate, this process is extremely energy-intensive and is therefore unattractive not only from economic aspects but also from ecological aspects.
In principle, dimethyl oxalate forms oxalic acid half-esters or oxalic acid, in accordance with reaction equations (7) and (3), by reaction with water present in traces, which originates from the preparation of methyl nitrite in the methyl nitrite reactor 3 of FIG. 2 and, because the separation there never takes place completely, is typically contained in reactant gas mixtures and therefore also in product gas mixtures of dimethyl carbonate preparation. Because of their acidity in extraction column 2, and in particular because of the higher temperature in methanol column 4 of FIG. 2, these products can convert the methanol present there into dimethyl ether in accordance with reaction equation (9). This process is autocatalytic, since a further equivalent of water is liberated with each reaction event, and in turn can react again with dimethyl oxalate. EQU H.sub.3 COOC--COOCH.sub.3 +H.sub.2 O.fwdarw.H.sub.3 COOC--COOH+CH.sub.3 OH (7) EQU H.sub.3 COOC--COOH+H.sub.2 O.fwdarw.HOOC--COOH+CH.sub.3 OH (8) ##STR3##
Furthermore, the oxalic acid half-ester formed in accordance with reaction equation (7) can decarboxylate to form methyl formate in accordance with reaction equation (10). EQU H.sub.3 COOC--COOH.fwdarw.HCOOCH.sub.3 +CO.sub.2 ( 10)
Industrially available carbon monoxide moreover contains small amounts of gaseous impurities which are inert under the dimethyl carbonate preparation conditions, such as, for example, hydrogen, methane and carbon dioxide, even after extensive purification.
The inevitable accumulation of volatile secondary components in the recycled circulating gas, whether they are those formed in the course of undesirable side reactions or those which the raw materials employed contain as impurities, requires removal of a corresponding content of the circulating gas from the circulation (purging). Although this is mentioned in principle in the process description of the Patent Application EP 523 728 cited, no information is given on the extent and treatment of the amounts of gas removed from the circulation. At all events, it is to be expected that both the economics and the ecology of the process will be impaired by this operation.
Beyond these considerations, the description of the process is inconsistent or defective, for example, at the following places:
A tube-bundle reactor comprising 6 tubes, the tubes of which have a diameter of 26.1 mm each and a length of 500 mm each, is thus described in column 11, lines 40 to 42 of Patent Application EP 523 728. Such a reactor has a maximum volume of 1.6 l. According to line 43 of Patent Application EP 523 728, however, this reactor is filled with 1.73 l of catalyst.
According to column 12, line 15 of Patent Application EP 523 728, 2.8 kg/hour of an absorption solution are removed at the bottom of the dimethyl carbonate extraction column (compare number 2 in FIG. 2 in this application, corresponding to number 2 in FIG. 1 of Patent Application EP 523 728) and are fed to the distillation column (compare number 4 in FIG. 2 in this application, corresponding to number 4 in FIG. 1 of Patent Application EP 523 728). According to column 12, lines 51 to 52 of Patent Application EP 523 728, however, this amount is 3.5 kg/hour.
According to column 13, lines 2 to 4 of Patent Application EP 523 728, a mixture which comprises dimethyl carbonate to the extent of 14.3% and dimethyl oxalate to the extent of 87.5% is removed from the bottom of the first distillation column (methanol distillation, compare number 4 in FIG. 2 in this application, corresponding to number 4 in FIG. 1 of Patent Application EP 523 728). However, in purely mathematical terms, this is not possible.
According to column 13, line 21 of Patent Application EP 523 728, 4.69 kg/hour of dimethyl oxalate are removed from the bottom of the second distillation column (dimethyl carbonate distillation, compare number 5 in FIG. 2 in this application, corresponding to number 5 in FIG. 1 of Patent Application EP 523 728). However, this amount is at least 0.6 kg greater than it can be according to the information in column 12, lines 15 to 16 or in column 12, lines 51 and 55 of Patent Application EP 523 728.
The methanol content of the gas which leaves the methyl nitrite synthesis reactor (number 3 in FIG. 2 in this application, corresponding to number 3 in FIG. 1 of Patent Application EP 523 728) and which, after carbon monoxide has been admixed, is fed into the dimethyl carbonate synthesis reactor (number 1 in FIG. 2 in this application, corresponding to number 1 in FIG. 1 of Patent Application EP 523 728) is determined by the exit temperature of the condenser at the top of the methyl nitrite synthesis reactor for a given pressure and given contents of the other gaseous components present in the total gas mixture. It is not a freely selectable parameter, but corresponds to the partial vapour pressure which is established under these conditions and, for Example 1 of the Patent Application EP 523 728 cited, is between 5.5 and 5.8% by volume. However, 1.8% by volume is mentioned in Example 1, column 11, lines 51 to 52 of the said patent application.
Finally, the data for the amount of water which is formed and removed from the circulation and results from the preparation of methyl nitrite (0.07 kg/hour) do not correspond to the amount which would be expected on the basis of the reaction yields of dimethyl carbonate and dimethyl oxalate described in Example 1 of the EP 523 728 cited, that is to say 0.14 kg/hour.
There was thus the object of discovering a process which is characterized by a lower expenditure of raw materials and energy, by a lower amount of by-products obtained and by a more effective and as far as possible simpler isolation and purification of the desired dimethyl carbonate than is described by the prior art. This object is achieved by the process according to the invention.
Various catalysts and catalyst types are described in the literature for the preparation of dimethyl carbonate carried out in accordance with reaction equation (1) by reaction of carbon monoxide with methyl nitrite over heterogeneous platinum metal supported contact catalysts in the gas phase.
Thus, for example, according to the scientific publication Cuihua Xuebao 10 (1), pages 75 to 78 (March 1989), catalysts which can be used are, for example, palladium(II) halides, preferably palladium(II) chloride, and in particular palladium(II) chloride which is fixed on active charcoal supports and is doped or modified with compounds of iron, lithium and/or copper, high selectivities and space/time yields of the desired dimethyl carbonate being obtained. Similar catalyst systems are described in the Patent Applications EP 425 197, EP 464 460, EP 503 091, EP 503 618 and EP 523 728. Catalysts of this type in general produce the desired dimethyl carbonate in a selectivity which is not completely satisfactory. Undesirable dimethyl oxalate is formed as a by-product in accordance with reaction equation (6). On the one hand, this is detrimental to the highest possible utilization of the raw materials employed, which is to be aimed for, and on the other hand it necessitates an additional separation expenditure in the course of the isolation and purification of the desired dimethyl carbonate. Furthermore, many catalysts of the type mentioned undergo a discharge of halide ions, specifically chloride ions, in general in the form of hydrogen halide formation, specifically hydrogen chloride formation, in the course of relatively long operating times.
This is associated, where appropriate, with a decrease in selectivity with respect to dimethyl carbonate formation and a drop in catalyst activity, although this can be avoided by addition of even small amounts of, for example, hydrogen halide, specifically hydrogen chloride, to the reactant gas mixture, as is described, for example, in the Patent Application EP 425 197 cited. Introduction of the small amounts of hydrogen halide, specifically hydrogen chloride, mentioned results in increased requirements on the materials from which the plant components which come into contact with the compound are to be produced. The formation of characteristic by-products, such as, for example, methyl chloride, which is formed in accordance with reaction equation (11) by the reaction, which runs in parallel with the conditioning of the catalyst, of hydrogen chloride with the methanol which is always present in small amounts in the reactant gas mixture is furthermore to be taken into account. Patent Application EP 565 076 demonstrates by way of example the severe deactivation phenomena (only 50 to 500 hours until the activity has disappeared virtually completely) which catalysts based on active charcoal supports undergo. A process is described there for batchwise regeneration of contact catalysts deactivated to this extent, which comprises sequential treatment of these catalysts with hydrogen and hydrogen halide at elevated temperatures. When realized industrially, such a process would require a constant starting up and running down of the production plant at intervals of a few hundred operating hours in the most favourable case, in order to allow regeneration campaigns. Alternatively, a double reaction procedure could also be conceived, which is operated in alternation between production and regeneration cycles. Another possibility would be to remove portions of the catalyst from the reaction zone batchwise or continuously, to regenerate them externally and to recycle them again to the reactor. In any event, the industrial solution to the deactivation problem would cause high additional costs and would be extremely unfavourable from economic aspects. It may thus be advantageous to resort to catalysts which show no deactivation or only an acceptably lower deactivation on the basis of the discharge of halide ions, specifically chloride ions, mentioned, even if such contact catalysts, such as are described, for example, in the Patent Applications EP 503 091, EP 503 618 and EP 523 728, show a rather more unfavourable selectivity due to the formation of dimethyl oxalate. EQU CH.sub.3 OH+HCl.fwdarw.CH.sub.3 Cl+H.sub.2 O (11)
To prevent the accumulation of by-products within an industrial circulatory process, portions to be specified of the circulating gas and the condensed reaction products, where these are not dimethyl carbonate itself, must be removed from the circulation (purged) continuously or discontinuously, preferably continuously.
New perspectives result for industrial realization with the discovery of new catalysts which allow the preparation of dimethyl carbonate by heterogeneously catalyzed reaction of methyl nitrite and carbon monoxide in the gas phase with considerably increased selectivities. Contact catalysts of this type, such as are described, for example, in Patent Applications EP 523 508, EP 438 676, EP 559 001, EP 558 996 and EP 581 240 have allowed only very little dimethyl oxalate as a by-product. For example, their activity and their DMC selectivity can be kept at a practically unchanged high level for a long time by continuous addition of very small amounts of hydrogen chloride to the reactant gas mixture.